Extending ER Diagrams (13)

CPSC 356 Database

Ellen Walker

Hiram College

(Includes figures from Database Systems: A Practical Approach to Design, Information and Management, by Connolly and Begg, © Addison Wesley 2010)

Basic ER Diagram Cannot Capture Specialization/ Generalization

• Examples– Doctors and Nurses are Medical Staff– Staff includes Medical staff, Secretarial staff,

Housekeeping staff, etc.

• Relationships might include Doctors, Medical Staff, and/or Staff

• ER diagram cannot represent the fact that Staff > Medical staff > doctors

• EER diagram can!

Entity Type Hierarchies

• An entity type E1 is a subclass of the entity type E2 when– Every entity instance of E1 is also an instance of

E2– Every attribute in E2 is also in E1

• The opposite of a subclass is a superclass• Subclasses are related to superclasses

through the IS-A relationship

When to Use?

• When you have an entity that needs to have extra / different attributes “sometimes”

• When you have two entities that overlap on many or most of their attributes

Subclass / Superclass Example

• Doctor is a subclass of medical-staff• Staff is a superclass of medical-staff

• Attributes – Staff - name, addr, salary– Med-staff - pager #– Doctor - license #

Staff

Medical-Staff Admin-Staff

Doctor Nurse

Attribute Inheritance

• Each entity type inherits all attributes of its superclass(es)– Staff has name, address, salary– Medical staff has {n,a,s} + beeper-number– Doctor has {n, a, s, b#} + license #

• In EER diagram, inherited attributes only appear once (in the highest superclass)

Generalizing Type Hierarchies

• Multiple superclasses for the same subclass– A student-employee is both a student and an

employee

• Overlapping subclasses– Subclasses of person are student and employee– There is at least one individual who is both a

student and an employee

• Non-overlapping subclasses are called “disjoint”

Specialization of Staff (13.2)

Extended ER (EER) Diagram

• IS-A relationships• Notation for disjoint (vs. overlapping)

subclasses– Disjoint = “or”, overlapping = “and”

• Notation for “covering”– Can someone be staff without being Medical or

administrative?– If so, “optional”; if not “mandatory”

More Staff Types (13.3)

Additional Semantic Relationships

• Aggregation and Composition– Collecting together related entities that represent ‘parts’ of a

‘whole’– Aggregation (non-exclusive subpart): parts can change parents– Composition (exclusive subpart): single parent, very strong

association - parts make no sense without parent– Examples:

• Branch (whole) has Staff (part) Aggregation

• Newspaper (whole) contains Story (part) Composition

• EER representation is diamond with arrow from “part”– Open for aggregation, closed for composition – (see 13.9, 13.10)